1. Field of the Invention
The present invention is directed to a system and a method for determining the distance from and the direction to an object.
2. Description of the Related Art
For determining the distance from an object, sensors are usually used which emit a signal and receive an echo of the signal which is reflected by the object. The distance from the object is determined based on the propagation time of the signal from the point in time when the signal is transmitted until the echo is received.
In addition to the distance from an object, the direction in which the object is located is also usually of interest. In particular for applications in the automotive field, the information concerning the direction of an object as well as the distance information are essential. Distance information and direction information for objects are also of crucial importance in robotics, for example for obstacle recognition in near-field navigation.
To determine the direction of the object, trilateration methods may be used, in which the direction of the object is determined based on the distances to the object measured by at least two sensors. The disadvantages of the trilateration methods lie essentially in the problem with assignment, and thus, the ambiguity in the distance estimation and position when there are multiple reflectors in the scene. These ambiguities may typically be resolved only via multiple measurements from various positions, and so-called tracking methods.
Alternatively, the transmission and/or reception characteristics of an individual sensor are often so greatly limited that the detection range of the sensor allows the lateral or vertical positioning of an obstacle. In this case it is disadvantageous that the transmission and/or reception characteristics must be greatly limited in order to achieve position accuracies which are relevant in practice, so that a large number of sensors is necessary for monitoring the detection range.
In addition to triangulation methods for direction determination, beamforming methods in particular may also be used. In active beamforming methods, the phases of multiple parallel active emitters are precisely tuned to one another in order to control the angle of reflection of the main lobe of the superimposition of the transmitted signals. In passive beamforming methods, the phase information of multiple receivers recording in parallel is used to reconstruct the angle of incidence of a received signal. In many beamforming methods, the size of an array, in particular the diameter of the array, of multiple parallel active emitters, or for a passive method, of the array of multiple parallel recording receivers, is important, since the size of the array determines the range of a transmitted signal as well as the angle separation capability for a received signal. In addition, the distance between the array elements, i.e., the receivers in passive beamforming or the emitters in active beamforming, is also important (element spacing). In most beamforming methods, the element spacing should be less than or equal to one-half the wavelength of the signal, since otherwise, so-called grating lobes, i.e., side lobes in the transmission or reception characteristics on the order of magnitude of the main lobe, may appear which may result in ambiguities in the detection.
As a result of the element spacing being less than or equal to one-half the wavelength of the signal, the size of the usable array elements is limited. For this reason, most practical applications for acoustic waves and ultrasound are currently found in medical technology or underwater applications, since these allow a longer wavelength in the medium as well as greater element spacings. In addition, the coupling of the medium to the elements is much more favorable in water and in tissue than in air.
Applications in air, such as for obstacle recognition in the automotive field or in robotics, for example, require very small array elements having a diameter, for example, of less than 1.7 mm at 100 kHz, due to the necessary small element spacing as a function of the frequency used. Due to the poor coupling of the emitters and receivers to the medium air, large emitter surface areas in the case of emitters, or small masses/large surface areas in the case of receivers, are required. However, large surface areas for individual array elements are not achievable due to the required small element spacing between the array elements.
A system in which a transmitter and multiple receivers are arranged in an array is described in U.S. Patent Application Publication 2008/0165620, for example. However, the receivers are only conditionally suited for application in the automotive field, since the described thin film piezoelectric emitter is susceptible to mechanical strain. In addition, the system described in U.S. Patent Application Publication 2008/0165620 is used in conjunction with trilateration methods. These methods allow much larger emitters and receivers than beamforming methods. Therefore, it is assumed that the sensor described in U.S. Patent Application Publication 2008/0165620 is not suited for beamforming methods.
Beamforming methods for ascertaining the distance from and the direction to an object are described in French patent document 2 817 973 and German patent application publication 10 2004 050 794, for example. A disadvantage of the method described in French patent document 2 817 973 is that a homogeneous linear array is assumed, and in particular the function of transmission is not separated from the function of reception. In the method described in German patent application publication 10 2004 050 794, large emitters are implicitly assumed in order to achieve a narrow emitter characteristic. Although this patent describes an active beamforming, the large emitter surface as well as the narrow emitter characteristic conflicts with use as an active emitter array. In addition, the two transmitting frequencies used conflict with the approach described in this claim.